Seeing as I’m trying to be a generally positive person and all round nice guy I really want to say ‘Hey, nothing’s impossible dude, shoot for the moon’ but the realist in me wants to add ‘then flail your arms uselessly as you end up adrift in the endless black void between the stars’…
Firstly, as others have mentioned, it’s well worth sorting out the legal side of the project in advance. It can seem a small thing to worry about now when everyone is champing at the bit to get down and dirty with the world building and project goodness, but better now than a year-18 months down the road when someone pulls out and tries to take half the project with them or stalls the development forever with a legal fight over who owns exactly what IP/code/whatever. Sadly it’s almost inevitable that at some point bad blood will happen among a team. People will come, people will go; make sure your project is safe from the petty vagaries of egos.
Once legals are done it’s project time, for now forget about world building or the whole 5 acts of glorious semi open wonder, that stuff’s largely suit. What you need first is the body to put it on! Concentrate on the various mechanics and systems you’ll definitely need. Wherever possible brainstorm with your group to work out small games that can be developed to flesh out these sub systems - These’ll serve a few purposes; they’ll give you the elation of getting small projects done and complete, they’ll raise your teams profile, help people flex and expand their skillsets and they could give a small revenue stream that’ll help pay for your ongoing costs such as software etc.
Stealth system? Make a top-down stealth-em-up. Fast twitch combat? Try a beat-em-up / street fighter clone. Branching conversation system? Pub quiz machine… and so on.
Of course you can’t forget entirely about the larger goal, you can develop the visual style and assets while you work on the smaller aspects and hit those short term achievable targets.